Government spokesman Panitan Wattanayakorn said Sunday the blast at a Rajdamri bus stop was intended to cause injries to people.
It was the first violence in Bangkok after the May chaos in which red shirts protesters burnt down many buildings in Bangkok.
This Big C branch and the CentralWorld Shopping Complex on the opposite site of the road, were burnt down during the riots.
Panitan said that it seemed like they placed the bomb at a place that would cause injuries to a number of people. They are quite serious about causing trouble, he said.